2. Antibodies
Proteins that recognize and bind to a particular
antigen with very high specificity.
Made in response to exposure to the antigen.
One virus or microbe may have several antigenic
determinant sites, to which different antibodies
may bind.
Each antibody has at least two identical sites
that bind antigen: Antigen binding sites.
Valence of an antibody: Number of antigen
binding sites. Most are bivalent.
Belong to a group of serum proteins called
immunoglobulins (Igs).
3. Antibody Structure
Monomer: A flexible Y-shaped molecule with
four protein chains:
2 identical light chains
2 identical heavy chains
Variable Regions: Two sections at the end of Y’s
arms. Contain the antigen binding sites (Fab).
Identical on the same antibody, but vary from
one antibody to another.
Constant Regions: Stem of monomer and lower
parts of Y arms.
Fc region: Stem of monomer only. Important
because they can bind to complement or cells.
5. Immunoglobulin Classes
I. IgG
Structure: Monomer
Percentage serum antibodies: 80%
Location: Blood, lymph, intestine
Half-life in serum: 23 days
Complement Fixation: Yes
Placental Transfer: Yes
Known Functions: Enhances phagocytosis,
neutralizes toxins and viruses, protects fetus and
newborn.
6. Immunoglobulin Classes
II. IgM
Structure: Pentamer
Percentage serum antibodies: 5-10%
Location: Blood, lymph, B cell surface (monomer)
Half-life in serum: 5 days
Complement Fixation: Yes
Placental Transfer: No
Known Functions: First antibodies produced
during an infection. Effective against microbes and
agglutinating antigens.
7. Immunoglobulin Classes
III. IgA
Structure: Dimer
Percentage serum antibodies: 10-15%
Location: Secretions (tears, saliva, intestine, milk),
blood and lymph.
Half-life in serum: 6 days
Complement Fixation: No
Placental Transfer: No
Known Functions: Localized protection of mucosal
surfaces. Provides immunity to infant digestive
tract.
8. Immunoglobulin Classes
IV. IgD
Structure: Monomer
Percentage serum antibodies: 0.2%
Location: B-cell surface, blood, and lymph
Half-life in serum: 3 days
Complement Fixation: No
Placental Transfer: No
Known Functions: In serum function is unknown.
On B cell surface, initiate immune response.
9. Immunoglobulin Classes
V. IgE
Structure: Monomer
Percentage serum antibodies: 0.002%
Location: Bound to mast cells and basophils
throughout body. Blood.
Half-life in serum: 2 days
Complement Fixation: No
Placental Transfer: No
Known Functions: Allergic reactions. Possibly
lysis of worms.